Implantable medical devices are used for a wide variety of medical conditions, such as for example, cardiac pacing and sensing, cardiac rhythm management, treatments for congestive heart failure, implanted defibrillators, and neurostimulation. Neurostimulation encompasses a wide range of applications, such as for example, pain control, nervous tremor mitigation, incontinent treatment, epilepsy seizure reduction, and vagus nerve stimulation for clinical depression. These implantable medical devices generally include an implanted pulse generator that generates electrical pulses or signals that are transmitted to a targeted tissue or nerves through a therapy delivery element, such as a lead with electrodes.
Controlled placement of the therapy delivery element is required for improved therapeutic efficacy or reduced side effects. Retaining the implanted therapy delivery element in the desired location also creates difficulties because the location may change over time as the patient moves. Anchors typically form a mechanical resistance lock that prevents the therapy delivery element from sliding around as the patient moves. In most configurations, anchors consist of a metal insert housed inside of a silicone sleeve and function by using suture knots to collapse the insert onto the lead. A variety of anchors are available to prevent the therapy delivery element from migrating away from a specifically selected stimulation site.
Clinicians inserting and anchoring leads typically prefer to perform the procedure rapidly, in a minimally invasive manner, and fix the lead in a manner that reduces the opportunity for the lead to migrate if practicable. Examples of some previous anchors are shown in U.S. Pat. No. 6,134,477 “Adjustable Medical Lead Fixation System” by Knuteson (Oct. 17, 2000); U.S. Pat. No. 5,484,445 “Sacral Lead Anchoring System” by Knuth (Jan. 16, 1996); and, U.S. Pat. No. 5,843,146. “Adjustable Medical Lead Anchor” by Cross, Jr. (Dec. 1, 1998).
U.S. Pat. No. 4,553,961 (Pohndorf et al.) discloses a typical suture sleeve with an outer elastomeric sleeve and an inner gripping structure. The lead is inserted though a lumen in the anchor. The gripping structure is radially compressed by the surgeon tying a suture material around the suture sleeve. The suture material causes the outer elastomeric sleeve to compress the inner gripping structure, which then collapses onto and grips the lead.
An issue with such anchors is that the elastomeric sleeve (typically silicone) may be too thick and prevents the insert from fully closing. In most cases, failure to secure the lead is due to the silicone absorbing most of the compressive force applied by the sutures, or the sleeve getting caught in the gaps of the insert as it closes. This problem will become a larger issue as leads become more flexible in the future. Essentially, as leads become more flexible they will tend to neck down when stretched, causing the overall diameter of the lead to decrease. Since these anchors function by a resistance lock, the holding force is greatly reduced as the lead necks down. As a result, anchors will have to collapse further in order to maintain a high holding force. If the sleeve interferes with compression of the insert, lead migration can result.